The principle of an inductive proximity switch is disclosed in EP-A-0492029 B1. In that proximity switch a, coil 11 is supplied with periodic unipolar voltage pulses having a period T. The useful signal is obtained by a suitable electronic circuit which responds to the voltage induced after the end of a transmitting current pulse in coil 11 by the decaying current which previously flows in the detected body, which consists of an electrically conductive material, due to the voltage induced therein by the transmitting current pulse.
The proximity switch known from EP-A-0492029 B1 has the following drawbacks:
The unipolar nature of the transmitting pulses implies that in each period T, the next transmitting pulse can only start when the current generated by the voltage induced in the detected body has almost completely decayed. Otherwise, an accumulation of currents results in the detected body which impairs the quality of the received signal. The mentioned waiting time results in a low clock frequency, thereby reducing the switching frequency of the device (according to a pertinent product standard EN 60947-5-2, Art. 2.4.3 the switching frequency is given by the number of switching operations of a proximity switch within a predetermined time interval).